The use of working casts with removable dies has become very common in dental practice. Using any one of various known techniques, a cast can be produced from an impression in which the dies of the teeth being worked on are removably positioned. Typically, dowel pins are utilized to removably couple the dies to the cast. In most cases tapered dowel pins are utilized to facilitate insertion of the removable dies into the cast.
After the dies have been properly formed and indexed in the cast, one of the important aspects is to be able to return the dies back to their exact position in a repeatable manner in the cast. Additionally, the dies must remain stable when positioned in the cast, even when the cast is inverted.
One technique that is generally used for forming working casts with removable dies has been generally referred to as the "one-pour technique". In this technique, dowel pins are positioned over an impression tray and maintained suspended over the tray. Die stone is poured into the impression tray to form the full cast of the teeth and dies of the prepared-teeth to be worked on. After completion, the stems of the dowel pins project from the die. In another technique additional stone is poured to form the working cast with the dowel stems extending into and through the cast. This is known as a double pour technique. When the model stone or gypsum hardens the model is cut to separate the teeth and the stems are tapped to separate each section with the dowel pin from the cast base.
In such cases, the dowel pins with the attached die are reinserted directly into the cast stone base. The continuous reinsertion of the dowel pin dies into the die cast stone necessitated by the subsequent technical procedures, may cause crumbling of the stone, do not permit easy seating of the dowel pin, and often prevents the die from being repositioned properly in the cast base. Additionally, since there is only a dowel pin for reinsertion, even though it may be tapered, and keyed in some form or another, the problem of orientation of the die into the cast may not be sufficiently specific to that the die can be inserted in various indefinite and exact positions thereby making the fabrication of accurate dental restorations difficult.
In order to properly orient the placement of removable dies into the cast, there has been developed the PINDEX System by the Whaledent Company. Such system is described in U.S. Pat. No. 3,704,519. In such system, holes are formed into the removable part of the die. Pins with bushings are then inserted and the pins with the bushings are pressed downwardly into the material forming the cast. The cast material sets with the bushings in place whereupon the dies may be removed by extracting the pins from the bushings. By using the two pins, for each die section, proper replacement of the die section into the cast is ensured.
The need for proper orientation of each die section is thus seen to be an important aspect of using removable dies with working casts. Both in the one-pour technique as well as in the PINDEX technique, such orientation is significant and the ability to easily remove and replace the removable die is important.